Are you conducting this transaction with Chief Amenities Officer Randall?
The issue may be that if you are not yet a Vault Citizen, then you should not be yet able to barter with Randall at all. Perhaps if you access the barter screen with the red button (instead of dialogue) some scripting restrictions still prevent you from trading. But this is just a guess, I've never heard of such a problem before.
You mentioned a Geiger counter , make sure it's tured off -shopkeepers might not accept activated items, this is a precaution to avoid getting them to buy primed explosives.
Doest this problem only affect the FAL? Can you trade other items without any constrictions? If the issue is only in regard to the FAL, then maybe some bizarre scripting glitch has allowed you to put a person's active weapon on the table, yet it does not allow you to buy it. But it's just a wild guess, not any better than the first.
Try out other combinations of items on both sides. If it doesn't help, you'll just have to find your guns elsewhere...